Effect pigments based on coated glass flakes

ABSTRACT

The present invention relates to effect pigments based on thin glass flakes and to a method for the production of such pigments. The resulting pigment can be used in any application for which pearlescent pigments have been heretofore used such as, for example, in plastics, paints, inks, cosmetic formulations, coatings including solvent or waterborne automotive paint systems, powder coatings, inks and agriculture foils.

The present invention relates to effect pigments based on thin glassflakes, to a method for the production of such pigments and their use inplastics, paints, coatings, powder coatings, inks, printing inks,glasses, ceramic products, agriculture foils, and in cosmeticformulations.

Since more than 40 years iridescent luster effects can be achieved usingso called nacreous or pearlescent pigments. Various alternativetechniques have been developed to create colour/lustre effects. The mostimportant, economic and common way to make these pigments is to coat aplatelet shaped carrier with high refractive substances like TiO₂,Fe₂O₃, SnO₂, ZrO₂, Cr₂O₃ or combinations of these or with alternatinglayers of high and/or low refractivity. So far the nearly exclusivelyused carrier is wet ground muscovite mica. It is cheap, readilyavailable and easy to cleave into smooth and very thin platelets. Thesecan be classified into any desired particle size distribution.Additionally, pigments based on mica are very stable towards chemical,mechanical or thermal treatment. But there are some disadvantages ofmica, since it is a natural product and shows inconsistencies fromsource to source and even from batch to batch. This has to be overcomeby adjusting the processing parameters. Natural mica contains impuritieslike quartz which makes the processing more complicated (abrasion ofgrinding tools, waste) or like Fe, Mn, Cu which makes the masstoneyellowish grey rather than white. When ground it yields plateletsshowing a broad distribution of thicknesses and diameters. This can bereduced by proper classification but will never lead to uniform shape.

In the past researchers tried to find corresponding alternatives to micawhich keep the advantages of it but did not contain the above mentioneddisadvantages.

A wide variety of other platy materials have been proposed as substitutefor mica in the patent literature. These include non-soluble inorganicmaterials such as glass, enamel, china clay, porcelain, natural stonesor other silicaceous substances, metal objects and surfaces of organicpolymer materials such as polycarbonate as disclosed for example in U.S.Pat. Nos. 3,123,485, 3,219,734, 3,616,100, 3,444,987, 4,552,593 and4,735,869. While glass has been mentioned as a possibility on manyoccasions, for instance in U.S. Pat. No. 3,331,699, commercialpearlescent products are not made using glass and experience has shownthat products made using glass as the platelet substrate have ratherpoor quality.

U.S. Pat. No. 3,331,699 discloses that glass flakes may be coated with atranslucent layer of particles of a metal oxide having a high index ofrefraction, such as titanium dioxide, provided there is first depositedon the glass flakes a nucleating substance which is insoluble in theacidic solution from which the translucent layer of metal oxide isdeposited. The patent does not mention the necessity of a smoothtransparent film, not particles, being necessary for qualityinterference pigments to be developed. The patent teaches that thenature of the glass is not critical, but that the presence of thenucleated surface is critical. It is further stated that there are onlya small number of metal oxide compounds which are insoluble in theacidic solution and capable of forming a nucleated surface on the glassflakes; tin oxide and a fibrous boehmite form of alumina monohydrate arethe only two such materials disclosed. As demonstrated in the examplesbelow, products prepared according to the teachings of this patent arepoor in quality.

U.S. Pat. No. 5,436,077 teaches a glass flake substrate which has ametal covering layer on which is formed a dense protective coveringlayer of a metal oxide such as titanium dioxide. In this patent, thenature of the glass is unimportant, the metallic coating provides thedesired appearance and the overcoating of the metal oxide is present toprotect the metallic layer from corrosive environments.

EP 0 912 640 B1 teaches the coating of thick flakes of C glass having afirst coating comprising iron oxide or rutile titanium dioxide thereon.The disadvantage of C glass is the limited thermal stability. Asdisclosed in the EP 0 912 640 B1 the rutile pigments prepared accordingto the corresponding examples were calcined at temperatures not higherthan 600° C. It is well known that the calcining temperature is ofessential importance for the stability of rutile pigments, especiallyfor outdoor applications. To yield sufficiently stabilized rutile TiO₂layers on a substrate calcining temperatures of at least 800° C. arerequested.

For the preparation of pearlescent pigments the transparency and thethickness of the platy substrate are very important. For the first timeEP 0 289 240 B1 discloses the manufacturing of extremely thin glassflakes at reasonable costs. According to the claimed process the glassflakes cannot be only made in any desired composition, e. g., from pureSiO₂, but also in any thickness tailored for the application wanted downto below 0.8 μm.

It is an object of the present invention to overcome the problems of theprior art and to provide novel effect pigments which have advantageousapplication properties. This object is achieved by the inventive effectpigments based on very thin glass flakes having the followingcharacteristics:

-   (1) thickness of the glass flakes ≦1.0 μm-   (2) high temperature and mechanical stability-   (3) smooth surfaces

The present invention relates to effect pigments based on glass flakeswith a thickness of ≦1.0 μm coated with one or more layers with a highand/or a low reflective index. The thickness of the glass flakes ispreferably ≦0.8 μm and especially ≦0.5 μm. Especially preferred are thinglass flakes with a softening point ≧800° C.

Glass can be classified for example as A glass, C glass, E glass, ECRglass.

For the present invention quartz glass is preferred but the productionof this glass is very expensive. Glass types which fulfill the featureof the requested softening point are quartz glass, and any other glasscomposition having a softening point of ≧800° C. Glass flakes whichfulfill the requirements are special glasses like e.g. Schott Duran orSupremax types The softening point in the present invention is defined,according to ASTM C 338 as the temperature at which a uniform fiber ofglass with a diameter of 0.55–0.75 mm and a length of 23.5 cm increasesits length by 1 mm/min when the upper 10 cm is heated at a rate of 5°C./min.

Suitable glass flakes preferably prepared according to EP 0 289 240 B1are characterized in that they contain an average particle size in therange of 5–1000 μm, preferably in the range of 5–150 μm. Preferred glassflakes have an average particle size in the range of 5–150 μm and athickness of 0.1–0.5 μm, preferably of 0.1–0.3 μm. The aspect ratio ofglass flakes is in the range of 10–300, preferably in the range of50–200.

The glass particles can be coated with one or more layers selected fromthe group consisting of metal oxides, metal suboxides, metal fluorides,metal oxyhalides, metals chalcogenides, metal nitrides, metal sulfides,metal carbides, or mixtures thereof.

Examples of suitable metal oxides are TiO₂, Fe₂O₃, TiFe₂O₅, Tisuboxides, Fe₃O₄, Cr₂O₃, Al₂O₃, SiO₂, ZrO₂, ZnO, SnO₂, CoO, Co₃O₄, VO₂,V₂O₃, Sn(Sb)O₂ or mixtures thereof. The TiO₂ layer tan be in the rutileor anatase modification, preferably the TiO₂ layer is rutile. Especiallypreferred are glass flakes coated with TiO₂ and/or Fe₂O₃.

Metal sulfide coatings on glass flakes are preferably selected fromsulfides of tin, silver, lanthanum, rare earth metals, preferablycerium, chromium, molybdenum, tungsten, iron, cobalt and/or nickel.

The glass flakes can be coated in the same way as e. g. mica based pearllustre pigments. Coatings with a metal oxide may be accomplished by anyknown methods, such as hydrolysis of a metal salt by heating or alkali,which deposits hydrated metal oxide, optionally followed by calcination.

In general, the procedure involves the dispersing of the thin glassflake particles and combining that dispersion with a precursor whichforms a hydrous metal oxide film coating on the flakes.

After the glass is dispersed in water and placed in an appropriatevessel, the appropriate metal salts are added. The pH of the resultingdispersion is maintained at an appropriate level during the addition ofthe metal salts by simultaneous feeding a suitable base, for examplesodium hydroxide, to cause precipitation of the hydrous metal dioxide onthe glass flakes. An aqueous acid, for example hydrochloric acid, tan beused for adjusting the pH. The coated platelets can, if desired, bewashed and dried before being calcined to the final pigment. Theprocedure is described in detail in U.S. Pat. No. 5,433,779 and in theGerman Patents 14 67 468, 19 59 998, 20 09 566, 22 14 545, 22 15 191, 2244 298, 23 13 331, 25 22 572, 31 37 808, 31 37 809, 31 51 343, 31 51354, 31 51 355, 32 11 602 and 32 53 017.

The effect pigments of the present invention are preferably prepared bywet-chemically coating of the glass flakes or by gas phase decompositionof volatile metal compounds (CVD, PVD) or electroless plating.

The effect pigments obtained in this way are characterized in that oneor more homogeneous layers enrobing the uniform thin glass flakes.

Preferred effect pigments of the present invention are given in thefollowing:

-   glass flake+TiO₂ (rutile)-   glass flake+Fe₂O₃-   glass flake+Fe₃O₄-   glass flake+TiFe₂O₅-   glass flake+Cr₂O₃-   glass flake+ZrO₂-   glass flake+Sn(Sb)O₂-   glass flake+BiOCl-   glass flake+Al₂O₃-   glass flake+Ce₂S₃-   glass flake+MoS₂

In general, the layer thickness ranges from 0.1 to 1000 nm, preferablyfrom 0.2 to 300 nm. The optical layer thickness will in general beadapted to the particular application. Preferred pigments are coatedwith one or two layers.

To enhance the light and weather stability it is frequently advisable,depending on the field of application, to subject the coated glassflakes to a surface treatment. Useful surface treatments include forexample the processes described in DE-C 22 15 191, DE-A 31 51 354, DE-A32 35 017 or DE-A 33 34 598, DE 40 30 727 A1, EP 0 649 886 A2, WO97/29059, WO 99/57204, U.S. Pat. No. 5,759,255. This surface treatmentfurther enhances the chemical stability of the pigments and/orfacilitates the handling of the pigment, especially its incorporationinto various application media.

The effect pigments of the present invention are advantageous useful formany purposes, such as the coloring of plastics, glasses, ceramicproducts, agriculture foils, decorative cosmetic formulations and inparticular coatings, especially automotive coatings, and inks, includingprinting inks. All customary printing processes can be employed, forexample offset printing, intaglio printing, bronze printing,flexographic printing. Furthermore they can be used as functionalpigments like conductive pigments, as magnetic pigments or to makemedia, for example plastics, board products and papers, laser-markable.

The effect pigments of the present invention are also advantageouslyuseful for these applications in admixture with filler pigments ortransparent and hiding white, colored and black organic and inorganicpigments and also with conventional transparent, colored and blackluster pigments based on metal oxide coated mica, TiO₂ flakes, SiO₂flakes or Al₂O₃ flakes and coated or uncoated metal pigments, BiOClpigments, platelet shaped iron oxides, or graphite flakes.

Additionally, the inventive pigment mixtures can contain organic orinorganic colourants, thixotropy agents, wetting agents, dispersingagents, water, organic solvent or solvent mixtures, etc.

The pigment mixtures of the invention are simple and easy to handle. Thepigment mixtures can be incorporated into the system in which it is usedby simple mixing. Laborious milling and dispersing procedures for thepigments are not necessary.

The coated glass flakes of the invention can be used for pigmentingand/or coating materials, printing inks, plastics, agricultural films,button pastes, for the coating of seed, for the colouring of food,coatings of foods, medicaments or cosmetic formulations. Theconcentration of the pigments in the system in which it is to be usedfor pigmenting is generally between 0.01 and 50% by weight, preferablybetween 0.1 and 5% by weight, based on the overall solids content of thesystem. This concentration is generally dependent on the specificapplication.

Plastics comprising the pigment mixture of the invention in amounts of0.1 to 50% by weight, in particular from 0.5 to 7% by weight, arefrequently notable for a particular brilliance.

In the coating sector, especially in automotive finishing, the glassflakes are employed in amounts of 0.5 to 10% by weight.

In the pigmentation of binder systems, for example for paints andprinting inks for intaglio, offset or screen printing, the pigment isincorporated into the printing ink in amounts of 2–50% by weight,preferably 5–30% by weight and in particular 8–15% by weight.

The invention likewise provides pigment preparations comprising coatedglass flakes, optionally effect pigments, binders and, if desired,additives, the said preparations being in the form of substantiallysolvent-free, free-flowing granules. Such granules contain up to 95% byweight of the inventive pigments. A pigment preparation in which theglass flakes of the invention are pasted up with a binder and with waterand/or an organic solvent, with or without additives, and the paste issubsequently dried and brought into a compact particulate form, e. g.granules, pellets, briquettes, a masterbatch or tablets, is particularlysuitable as a precursor for printing inks.

The present invention therefore also provides formulations containingthe pigments of the invention.

In order to further illustrate the invention, various non-limitingexamples are set forth below. In these, as well as throughout thebalance of this specification and claims, all parts and percentages areby weight and all temperatures are in degrees centigrade unlessotherwise indicated.

EXAMPLES Example 1 (Rutile Silver Pigment) Prior Art

200 g flakes of E-glass (thickness: 5 μm; diameter: 10–40 μm; specificsurface area: approximately 0.2 m²/g) are suspended in 2 l of deionizedwater. With vigorous stirring the suspension is heated to 80° C. Afterhaving adjusted the pH to 2.0 with dilute HCl a first layer of SnO₂ isprecipitated onto the glass flakes by feeding a solution of 3 g SnCl₄×5H₂O (in 10 ml conc. HCl plus 50 ml of DI water) into the suspensionunder simultaneous neutralization with 10% NaOH over a period of about 1h. To complete the reaction the suspension is kept stirring for another15 min. The coating is continued by adjusting the pH to 1.8 with diluteHCl and then titrating 63.5 ml of TiCl₄-solution (400 g TiCl₄/DI water)against 10% NaOH over a period of about 3 h. Having reached the desiredlayer thickness the coating is stopped, stirred for another 15 min,filtered, washed with DI water and dried. After calcination at 800° C.for 30 min a silverwhite rutile pigment is yielded.

Example 2 (Rutile Silver Pigment)

200 g flakes of E-glass (thickness: 0.5 μm; diameter: 10–40 μm; specificsurface area: approximately 1.7 m²/g) are suspended in 2 l of DI water.The coating is carried out the same way as in example 1. Titrationperiods keep the same, only the amounts of solutions are adjusted to thedifferent base material:

5 g SnCl₄×5 H₂O (in 15 ml conc. HCl plus 75 ml DI water)

196.3 ml of TiCl₄ solution (400 g TiCl₄/l DI water)

After calcination at 800° C. for 30 min a brilliant silverwhite rutilepigment is yielded.

Examples 3 and 4 (Rutile Interference Green Pigment)

The coatings are started in the same ways as for examples 1 and 2, onlythe amount of TiO₂ (400 g TiCl₄/l DI water) coated was increased toyield green interference type pigments. The further steps are againcarried out as described in examples 1 and 2.

To evaluate their properties draw down cards are made from all of thesepigments. Additionally sprayed panels were prepared. The pigments basedon glass flakes with a thickness of 5.0 μm and 0.5 μm are checked fortheir coloristics and their visual performance.

Results:

draw downcards and panels pigments with a thickness of 5.0 μm showedvery weak performance making a “dilute” impression. For the silverlittle brilliance and little hiding effect was observed while for thegreen a dull colour and little brilliance are found;

a little better is the effect in the sunlight as some larger flakes showa kind of sparkle;

completely different is the situation with a thickness of 0.5 μmpigments: They exceeded comparable mica pigments regarding brilliance,lustre and (for the green) chroma;

particularly impressing is the high transparency combined with clearcolor;

looking with the microscope shows that the surfaces of all pigments arewell and uniformly coated and that the differences result mostly fromthe imperfect orientation of the 5 μm particles;

1. An effect pigment comprising a glass flake with a thickness of ≦1.0μm coated with one or more layers, and the softening point of the glassflake being ≧800° C.
 2. An effect pigment according to claim 1, whereinthe glass flake is coated with one or more layers of metal oxide(s),metal suboxide(s), metal oxyhalide(s), metal fluoride(s), metalchalcogenide(s), metal nitride(s), metal sulfide(s), metal carbide(s),or a mixture thereof.
 3. An effect pigment according to claim 2, whereinthe metal oxide is TiO₂, Fe₂O₃, TiFe₂O₅, a Ti suboxide, Fe₃O₄, Cr₂O₃,Al₂O₃, SiO₂, ZrO₂, ZnO, SnO₂, CoO, Co₃O₄, VO₂, V₂O₃, Sn(Sb)O₂ or amixture thereof.
 4. An effect pigment according to claim 3, wherein theglass flake is coated with TiO₂, Fe₂O₃ or a mixture of TiO₂ and Fe₂O₃.5. An effect pigment according to claim 3, wherein TiO₂ is present andis in the rutile modification.
 6. An effect pigment according to claim2, wherein the coating is a metal sulfide selected from sulfides of tin,silver, lanthanum, rare earth metals, chromium, molybdenum, tungsten,iron, cobalt and/or nickel.
 7. An effect pigment according to claim 1,wherein the glass flake is coated with one or two layers.
 8. A method ofpreparing an effect pigment according to claim 1 which comprises coatingthe glass flake by wet chemical coating, by chemical or physical vapordeposition or electroless plating and optionally calcining the coatedglass flake.
 9. A plastic, coating, powder coating, paint, ink, printingink, glass, ceramic product, agriculture foil or cosmetic formulationcomprising a base material and an effect pigment of claim
 1. 10. Acomposition comprising a base material and a conductive pigment,magnetic pigment or dopant for the laser-marking of paper or plastics,wherein said pigment is of claim
 1. 11. A formulation comprising a basecomposition and an effect pigment according to claim
 1. 12. An effectpigment of claim 1 wherein said glass flake comprises C or E glass.